Since polyamide resins have excellent properties in heat resistance, rigidity, strength, oil resistance, coating properties, etc., they have widely been used in the fields of automobile parts, electrical and electronic parts as engineering resins. However, the polyamide resins not only involve difficulties and points to be improved in moldability, impact resistance, water resistance, and chemical resistance but also have a high specific gravity and are high in production cost as compared with, for example, polyolefin resins.
In order to improve these drawbacks, a thermoplastic resin composition having a good balance among physical properties such as moldability, rigidity, heat resistance, impact resistance, scratch resistance, oil resistance, chemical resistance, and water resistance and being excellent in uniformity of the appearance and smoothness, formed from polypropylene and a polyamide resin, the both of which are essentially incompatible with each other, by blending an epoxy group-containing copolymer with a polypropylene resin selected from modified polypropylene and a modified polypropylene/polypropylene composition and a polyamide resin to disperse them with good compatibility, is proposed as described, for example, in JP-A-61-64741 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, compositions obtained by blending such a polyamide resin with other resins such as polypropylene resins are not always satisfactory because the coating properties, adhesiveness, and printability which are excellent characteristic features inherent to the polyamide resin are likely lowered. Thus, such a defect is a hindrance in developing new practical applications.
On the other hand, in order to improve the compatibility between a resin having inferior coating properties and a coating composition, a single primer layer is usually used as an interlayer therebetween. However, since not only the cost of the final product is increased owing to a high cost of the primer itself and the addition of one new step, but also an environmental problem is provided by the treatment of the waste solution of the primer, etc., a further improvement has been desired.
Moreover, it is usually practiced well to wash the surface of a resin article with a vapor of a halogenated hydrocarbon solvent such as 1,1,1,-trichloroethane before coating a primer. However, the use of 1,1,1-trichloroethane is being restricted by the ozone problem which is one of the recent environmental measures of the earth.
As a surface pre-treatment for carrying out coating, printing, adhesion, etc., a sand blast treatment, a chromic acid mixture treatment, a corona discharge treatment, a plasma treatment, a surface functional group-imparting method, a surface photografting method, etc., are proposed. However, satisfactory results are not yet obtained in any of these methods from the standpoints of environment, performance, etc.
As described above, the resin composition containing a polyamide involves difficulties in the coating properties, and the use of a primer and the surface pre-treatment for improving the coating properties have various problems from the standpoints of environment, coating performance, etc. Accordingly, the development of resin compositions containing a polyamide, to which coating, adhesion, printing, etc. can be applied well without need of the above-described surface pretreatment step, has become a large technical theme.